Aluminum purification process

ABSTRACT

1. A PROCESS FOR PREPARING A PURIFIED ALUMINUM, WHICH PROCESS COMPRISES THE STEPS OF; DECOMPOSING AN ALUMINUM ALKYL TO DEPOSIT SOLID PARTICLES OF METALLIC ALUMINUM CONTAINING A SIGNIFICANT AMOUNT OF ALUMINUM CARBIDE, COMPACTING THOSE PARTICLES ALONG WITH PARTICLES OF SOLID FLUX INTO PELLETS FOR METLING, THE FLUX BEING AN ALUMINUM-TREATING FLUX THAT WHEN MOLTEN IS LESS DENSE THAN THE MOLTE ALUMINUM BUT MORE DENSE THAN OR ABOUT EQUAL IN DENSITY TO THE ALUMINUM CARBIDE CONTAINED IN THE MOLTEN ALUMINUM, MELTING THE PELLETS TO FORM A BODY OF MOLTEN ALUMINUM COVERED BY A LAYER OF MOLTEN FLUX, BUBBLING A CHLORINE-CONTAINING GAS THROUGH THE BODY OF MOLTEN ALUMINUM TO HELP REMOVE THE ALUMINUM CARBIDE FROM THE ALUMINUM AND CARRY THAT CARBIDE TO THE FLUX LAYER, REMOVING SOME OF THE PURIFIED ALUMINUM AND SOME OF THE CARBIDE-CONTAINING FLUXTHUS PRODUCED, AND USING THE REMAINING PURIFIED ALUMINUM AND REMAINING FLUX AS A HEEL FOR THE MELTING OF ADDITIONAL PELLETS.

United States Patent 3,846,122 ALUMINUM PURIFICATION PROCESS Alex R. Valdo, Baton Rouge, La., assignor to Ethyl Corporation, Richmond, Va.

No Drawing. Continuation-impart of abandoned application Ser. No. 26,751, Apr. 8, 1970. This application Aug. 2, 1971, Ser. No. 168,363

Int. Cl. C22b 21/06 U.S. Cl. 7568 C 2 Claims ABSTRACT OF THE DISCLOSURE A metallurgical process wherein aluminum carbide or other carbides are substantially removed from aluminum prepared via the pyrolysis of alkyl aluminum compounds, by treating the aluminum in molten form with a flux such as cryolite and/ or mixtures of sodium chloride potassium chloride, aluminum fluoride and sodium fluoride.

CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of US. application Ser. No. 26,751, filed Apr. 8, 1970 and subsequently abandoned.

BACKGROUND OF THE INVENTION Field of the invention The present invention is in the field of metallurgy and relates in particular to the manufacture of non-ferrous metals such as aluminum.

Description of the prior art Processes for the production of aluminum by thermal decomposition of pyrolysis of alkyl aluminum compounds have been suggested by K. Ziegler et al., US. Pat. No. 2,843,474 and by Ikeda et al., US. Pat. No. 3,154,407. In these processes, however, the actual production of aluminum is generaly accompanied by the formation of a sufficiently large amount of aluminum carbide to necessitate the purification of the aluminum by a subsequent treatment.

It is widely known in the manufacturing industry that the inclusion of a considerable amount of aluminum carbide in aluminum deleteriously aflfects the processability, corrosion resistance and other characteristics of the product aluminum. One solution of the carbide problem is taught by Tanaka et al., US. Pat. No. 3,170,787, wherein by regulating the rate of decomposition of the alkyl aluminum compound, the amount of carbide formed is claimed to be controlled to an acceptable level. Such a process requires a high degree of skill and is difficult to carry out effectively.

The problem of removal of carbide from aluminum metal is unique to a process based on pyrolysis of alkyl aluminum compounds. Carbide contamination is not a problem with aluminum metal prepared via the conventional Hall electrolytic process. The problem of carbide contamination is aggravated by the manner in which the carbide is formed. In the pyrolysis of alkyl aluminum compounds, aluminum carbide is formed simultaneously with aluminum metal, which produces a product in which the carbide is very intimately mixed with the aluminum, possibly even on a molecular level. On the other hand, aluminum from other sources of manufacture usually contains surface impurities such as oxides, which are more easily removed. The intimate nature of the carbidealuminum mixture makes more formidable the problem of carbide removal from aluminum derived from the pyrolysis of alkyl aluminum compounds.

The present invention is directed to a process wherein these aluminum carbides or other carbides can be substantially removed With comparative ease.

3,846,122 Patented Nov. 5, 1974 "ice SUMMARY OF THE INVENTION DESCRIPTION OF THE PREFERRED EMBODIMENT In the preferred embodiment of the invention, the impure aluminum metal or alloy is added to a furnace at 700 C.900 C. which contains a heel of molten pure aluminum and a molten flux phase. The impure material is added below the surface of the molten flux in order to minimize oxidation of the impure aluminum during melting. As the impure aluminum melts it coalesces into molten globules of metal which are subsequently dissolved in the heel of pure metal. The aluminum carbide, being insoluble in and less dense than the aluminum, slowly rises out of the molten metal and is transported to the flux phase. Gentle agitation of the charge by periodic rabbling or bubbling of an inert gas (such as nitrogen or argon) enhances the transfer of the carbide and other non-metallic impurities in the aluminum to the flux phase. Aluminum oxide, a common impurity of aluminum metal, is also effectively removed by this treatment.

When adequate separation and purification of the aluminum metal has been achieved, the furnace is tapped and a quantity of pure aluminum approximately equal to that added as impure aluminum is removed from the furnace. The aluminum left in the furnace is used as the molten heel for the next addition of impure aluminum (or alloy). Similarly, as the concentration of aluminum carbide (and oxide) and other non-metallic impurities builds up in the flux phase, all or part of the used flux can be removed from the furnace and an equivalent amount of fresh flux added. A very convenient way of adding fresh flux is by premixing it with the impure aluminum powder and compacting the mixture into pellets.

Preferred flux compositions are those which contain sodium chloride, sodium fluoride and aluminum fluoride or cryolite. Especially good results have been obtained with fluxes containing, by weight percent, 60% NaCl40% Na AlF and 51% NaCl35% NaF14% AlF Satisfactory results are obtained with other flux compositions. Some compositions, by weight percent, which gave satisfactory results are:

(a) 40% NaCl40% KCl20% Na AlF (b) 47.5% NaCl47.5% KC15%Na A1F (c) 45% NaCl45% KCl-10% Na AlF (d) 50% NaCl50% NaF The ideal flux is one which melts at a temperature slightly above the melting point of pure aluminum, and is less dense than aluminum metal but more dense than, or has about the same density as aluminum carbide. The

molten flux should also exhibit good fluidity at tempera ture. For best results, it is preferred to operate at a temperature below 900 C., and it is especially preferred to operate below 800 C. in order to minimize flux losses via volatilization and suppress any contamination of the aluminum by unwanted side reactions, such as aluminum nitride formation. The preferred amount in general is about 1 pound of flux per pound of carbide to be removed from a given amount of aluminum. Other ratios may be used, dependent on the composition of the particular sample which is to be purified. For economic reasons, it is generally desired to use as little flux as possible, consistent with satisfactory carbide removal and good aluminum metal recovery.

A subsequent or simultaneous treatment of the molten aluminum with chlorine can be used in conjunction with the salt flux to remove certain metal impurities from the aluminum, such as magnesium or sodium. The chlorine has a dual effect. By chemical reaction it removes certain impurity metals by converting them to their respective chlorides; simultaneously, the sweeping action of the gas enhances the transfer of non-metallic impurities (carbides and oxides) to the flux phase.

Some examples of alkyl aluminum compounds from which the aluminum or aluminum powder may be prepared are triethylaluminum, tri-n-propylaluminum, triisobutylaluminum, diethylaluminum hydride, di-n-propylaluminum hydride and diisobutylaluminum hydride. Mixtures of alkyl aluminum compounds are also suitable for the preparation of aluminum.

A number of studies have been made to illustrate the efiectiveness and/ or efficiency of the present metallurgical process.

GENERAL PROCEDURE Aluminum powder prepared via the decomposition of tripropyl aluminum was examined to establish a base line. The aluminum produced by such decomposition had a relatively high aluminum carbide (Al,,C content, running from about 0.33 to about 8.21 percent by weight. The aluminum powders were divided into several samples, and each sample was then compacted using 50 tons to 80 tons of ram pressure. A quantity of pure aluminum and flux were melted together and the impure aluminum compacts were added to the molten charge. Some samples were treated with chlorine gas for a predetermined amount of time. Subsequently the melts were stirred, skimmed and cast into discs. Drillings from the discs were then analyzed for carbide content by a Gas EvolutionVPC Method. Identical samples of the aluminum were prepared in the same manner except that no fiux was used. It was found that the cover flux of cryolite also promoted melting as it was discovered that large chunks of aluminum powder (0.150 inches thick X 0.500 square inches area) would not melt without a suitable cover flux. A control sample of high purity primary aluminum was also run. Because of the small scale of the experiments, and large surface to mass ratios, a large excess of flux was employed in the laboratory tests. Much smaller quantities of flux would be required in a commercial process. The results of these tests are set forth hereinafter.

EXAMPLE I The foregoing General Procedure was followed with the aluminum powder having been prepared by the decomposition of tripropyl aluminum. Cryolite was used as the fluxing material and in one run chlorine gas was added. From the results in Table I hereinafter, it is readily seen 1 Control sample, high-purity primary aluminum.

EXAMPLE II Following the foregoing General Procedure, a number of tests were run to determine the effectiveness of a salt flux for removing aluminum carbide (Al C from aluminum powder prepared by pyrolysis of tripropylaluminum (TNPA) in oil, triisobutylaluminum (TIBA) in oil and tripropylaluminum (TNPA) neat. The A1 0 contents of the starting samples varied from 1.6% to 7.5%. The flux used was a mixture by weight percent of 45% NaCl, 45 KCl and 10% cryolite. The results of these tests are set forth in Table II hereinafter.

A number of samples of aluminum powder obtained from pyrolysis of tripropylaluminum (TNPA) in oil were melted in virgin aluminum and flux and various flux compositions were tested. The runs were made in an electric resistance furnace. Plumbago crucibles were used for melting. The soak temperature was actual melt temperature, and in most cases the soak temperature was 50 C. above the eutectic temperature of the flux. The temperature was measured with an accuracy of 10 C. to +15 C. The flux and virgin aluminum were melted. The aluminum compacts were added as the last item and all ingredients were reheated to soak temperature.

The starting aluminum powder had a carbide content of 1.57 weight percent. Carbide analyses were made on the aluminum phase recovered from melting aluminum powder in the presence of the various fluxes on six of the runs. All of these showed that the salt flux treatment was eifective for removing carbon. The results are depicted in Table III hereinafter.

TABLE III [Aluminum powder melting, flux treatment] Wt. of powder plus Wt., Run Type and wt. virgin Time, hr. and percent number of flux Al, g. Temp, C. A140:

13 Starting powder 1. 57 A, 454 g 100+200 1 800 ,400 g 102+20o 15 700 0 C, 400 g. 102+200 1 750 0 B, 400 g. 108. 7-1-200 3 700 0. 01 D, 400 g. 100. 2+200 1 750 B, 400 g 100+200 -1 -700-750 20 D, 400 g 107. 7+200 7 Flux composition:

A. 50 wt. percent NaF +50 wt. percent NaCl.

1 1Nwt.( .J 1percent; AlFa wt. percent NeF +51 wt. percent 35 wt. percent AlFa +65 wt. percent NaCl.

C. D. 40 wt. percent N aaAlFu +60 wt. percent NaCl.

EXAMPLE IV Additional samples of aluminum powder obtained from pyrolysis of tripropylaluminum (TNPA) and triethylaluminum (TEA) were melted with different fluxes similarly to the procedure of Example III. The starting aluminum powder obtained from TNPA pyrolysis had a carbide content of 4.23 weight percent. The starting aluminum powder obtained from TEA pyrolysis had a carbide content of 8.21 weight percent. In all runs, no carbide was detected. The results are tabulated in Table IV hereinafter.

TABLE IV [Aluminum powder melting, flux treatment] Wt. of Al pow- Time, der b hr.

Run

Type and wt. number of flux Temp, C.

21 Starting Al powder, D TNPA pyrolysis.

H 400 grams flux used in all experiments. Flux compositions, wt. percent:

B. 14 All; NaF +51 NaCl. D. NaaAlFs NaCl.

b 200 grams virgin aluminum used in all experiments.

The foregoing disclosure and description of the invention is illustrative and explanatory thereof and various changes may be made within the scope of the appended claims without departing from the spirit of the invention.

What is claimed is:

1. A process for preparing a purified aluminum, which process comprises the steps of:

decomposing an aluminum alkyl to deposit solid particles of metallic aluminum containing a significant amount of aluminum carbide, compacting those particles along with particles of solid flux into pellets for melting, the flux being an aluminum-treating flux that when molten is less dense than the molten aluminum but more dense than or about equal in density to the aluminum carbide contained in the molten aluminum,

melting the pellets to form a body of molten aluminum covered by a layer of molten flux,

bubbling a chlorine-containing gas through the body of molten aluminum to help remove the aluminum carbide from the aluminum and carry that carbide to the flux layer,

removing some of the purified aluminum and some of the carbide-containing flux thus produced, and using the remaining purified aluminum and remaining flux as a heel for the melting of additional pellets.

2. The process of claim 1 in which the flux is a fluo- References Cited UNITED STATES PATENTS 3/1961 Grunert et al 68 RX 3/1962 Kurfman 7568 RX 8/1948 Smith, Jr. et a1 7593 E 11/1970 Copeland 7568 X 3/ 1972 Brondyke et al. 7593 AC 3/ 1972 Derham et a1 7568 R 9/ 1968 Toyoshima et al. 7568 CX 3/ 1907 Roberts 7593 AC 3/ 1934 Bonsack 7568 R 6/ 1939 Comstock 7568 RX 8/1939 Junker 7565 3/ 1940 Lindenberger 7568 8/ 1958 Bunbury et a1 7568 X 6/ 1961 Foster et al 7568 11/1971 Hess 7568 R FOREIGN PATENTS 1964 Great Britain 7568 R 1966 Great Britain 7568 C 1927 Great Britain 7568 R P L. DEWAYNE RUTLEDGE, Primary Examiner M. I. ANDREWS, Assistant Examiner PO-ww UNITED STATES PATENT'GFFICE a Y 5/69 r v CERTIFICATE F "cannons;

Patent No. I 3 ,8 l -6 'l22 Dated; November. 7

Inventor(s) I Alex V l-d0 It is certified'that error appears in the aboveddentifie patent and that said Letters Patent are hereby corrected as below-:-

Column 1, in the Heading and Cross-Reference reads 7 "26,751, filed April 8, 1970", should read 26,75l'and 26,752, both filed April 8, 1970 Column 2, line 54 reads "gave", should read give Column 4, Table III Run 17, last column, reads "0.01", should read 0.018

Column 4, Table III, Runs l8, 19, 20, last column, the

zero was omitted.

Signed and sealed this 28th day of January 1975.

(SEAL) Attest:

C. MARSHALL DANN Commissioner of Patents McCOY M. GIBSON JR. Attesting Officer 

1. A PROCESS FOR PREPARING A PURIFIED ALUMINUM, WHICH PROCESS COMPRISES THE STEPS OF; DECOMPOSING AN ALUMINUM ALKYL TO DEPOSIT SOLID PARTICLES OF METALLIC ALUMINUM CONTAINING A SIGNIFICANT AMOUNT OF ALUMINUM CARBIDE, COMPACTING THOSE PARTICLES ALONG WITH PARTICLES OF SOLID FLUX INTO PELLETS FOR METLING, THE FLUX BEING AN ALUMINUM-TREATING FLUX THAT WHEN MOLTEN IS LESS DENSE THAN THE MOLTE ALUMINUM BUT MORE DENSE THAN OR ABOUT EQUAL IN DENSITY TO THE ALUMINUM CARBIDE CONTAINED IN THE MOLTEN ALUMINUM, MELTING THE PELLETS TO FORM A BODY OF MOLTEN ALUMINUM COVERED BY A LAYER OF MOLTEN FLUX, BUBBLING A CHLORINE-CONTAINING GAS THROUGH THE BODY OF MOLTEN ALUMINUM TO HELP REMOVE THE ALUMINUM CARBIDE FROM THE ALUMINUM AND CARRY THAT CARBIDE TO THE FLUX LAYER, REMOVING SOME OF THE PURIFIED ALUMINUM AND SOME OF THE CARBIDE-CONTAINING FLUXTHUS PRODUCED, AND USING THE REMAINING PURIFIED ALUMINUM AND REMAINING FLUX AS A HEEL FOR THE MELTING OF ADDITIONAL PELLETS. 